Optical fiber production generally involves drawing the fiber, which usually is of silica glass, and then applying a dual layer of coating materials to the fiber. A first layer typically comprises a relatively soft material and the second layer typically comprises higher modulus curable polymeric material for maintaining high strength and abrasion resistance. Each fiber thus coated must be capable of withstanding, over its entire length, a maximum stress level to which the fiber will be exposed during installation and service. A single fiber failure can result in the loss of several hundreds of circuits. The coating, dual or otherwise, should function to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber, which could weaken it or even affect its transmission properties. Also the coating shields the fibers from surface abrasion, which could occur as a result of subsequent manufacturing processes and also handling during installation.
Optical fibers are usually coated during a wet coating process which typically involves drawing the fiber through a reservoir of liquid polymer material and then curing the liquid polymer to harden it by exposure to curing radiation, such as, for example, ultra-violet light. In the dual coating process, the coatings are applied in tandem or simultaneously (within the same applicator or die assembly). The tandem arrangement applies a first coating layer which is then cured, and then the second coating layer is applied and cured. In the simultaneous dual coating arrangement, both coats are applied after which they are cured. In both cases, the primary coating is typically a low modulus material and the second coating is a relatively high modulus material. During the wet coating process, air bubbles may become entrained between the fiber and the first or primary and secondary layers, or within the actual layers themselves. An air/coating interface can become unstable at higher speeds which leads to the formation of bubbles. Such bubbles give rise to a number of problems. Bubbles can cause losses in signal transmission by, for example, causing inhomogeneity of the modulus near the glass surface which can cause mechanical distortion of the fiber. In addition, bubbles can weaken the mechanical strength of the coated fiber.
There has been, and continues to be, increasing emphasis on fiber waveguide draw speeds. Much effort has been expended on increasing fiber velocity in the coating process while avoiding the formation of bubbles in the coating layers. In U.S. Pat. No. 4,246,299 of Ohls, a fiber is passed through an applicator having a die body that defines a small, vertically oriented, longitudinally tapered passage having a reservoir disposed about it. A series of radial ports provide fluid communication between the reservoir and the passage. Turbulence within the coating material, which causes entrapment of air bubbles, is reduced by maintaining the level of coating material in the passage. In U.S. Pat. No. 4,374,161 of Geyling et al. there is shown a coating arrangement wherein the fluid coating material is directed radially toward the fiber. The passage diameter for the fiber is large enough to prevent contact with the fiber, while the pressure of the fluid coating material is high enough to substantially prevent air from entering the applicator. In U.S. Pat. No. 4,480,898 of Taylor, there is shown a dual coating applicator having a die that provides for formation of a gap between the die and the first coated layer. A second die is located at the exit of the first die, with the second coating material flowing through a narrow passage between the first and second dies. The second die also provides for a gap so that the second layer is applied at a free surface at the point of contact with the first layer. This arrangement has been found to eliminate instabilities and coating non-uniformities at increased speeds.
Despite the numerous arrangements for reducing turbulence at high fiber velocities, it has, thus far, been extremely difficult to achieve uniform and consistent results with fiber velocities greater than 14 m/sec. In addition, the coating operation itself, in the foregoing arrangements, requires an operator to monitor and continuously adjust fluid pressure, and to adjust orientation of the die, for example, to insure coating concentricity. Thus, the coating processes of the prior art are susceptible to a great deal of improvement.